The present invention refers to a method of making a multi-layer paper web. It further refers to a multi-layer paper web having a three-dimensional structure. Especially it refers to the production of tissue products such as toilet and kitchen paper, paper towels, handkerchiefs, wiping material and the like.
It is very common to laminate two or more tissue paper plies in order to produce the final tissue product. Herewith a more flexible and softer tissue product is obtained as compared to if one single ply with a corresponding thickness and basis weight had been produced as for the laminated product. The absorbent capacity and the bulk are moreover improved.
The lamination of two or more tissue plies is often made by means of gluing. A mechanical embossing of the plies is also often performed before they are glued together. It is further known to laminate two plies only by means of a mechanical embossing, at which a mechanical joining of the plies occur in the embossing sites.
Through for example EP-A-796 727 it is known to first emboss two paper plies in a three dimensional structure with alternating raised and recessed portions, after which glue is applied to one of the plies and the two plies are joined in a press nip between two embossing rolls, so that the raised portions of the respective plies are glued to each other. A similar embossing procedure is shown in EP-A-738 588, according to which the glue also has a colouring effect.
U.S. Pat. No. 3,414,459 discloses laminating of tissue plies by a combined embossing and gluing procedure. The embossing is of so called foot-to-foot type according to which the raised protuberances of the embossed tissue plies are joined together. In U.S. Pat. Nos. 3,555,907 and 3,867,225 there are also disclosed a combined embossing and gluing process, but where the embossing is of so called nested type according to which the raised projections of one tissue ply will rest in and be joined to the depressions of the opposite ply.
There would however from a manufacturing point of view be a general advantage to be able to make a single ply tissue product. The downtime of the converting line would be considerably reduced and the speed of the converting machine could be increased. There are however difficulties to envisage process solutions for single ply tissue that would render the desired product properties in terms of softness and absorbency.
One way of achieving a single ply tissue product with possibly acceptable properties would be to join two or more individual layers in the wet state in the paper machine before the paper is pressed and dried, while striving to optimize the fibre structure and fibre properties in the individual layers. Two layers can be joined together in the wet state through several operations. One is to use a so called multilayer headbox, another is to form a second layer on top of a previously formed first layer and a third possibility is to couch together two separately formed layers.
There is however considerable doubt that any of the above mentioned methods of joining the wet layers would produce the required softness and absorbency to replace a multi-ply tissue. The reason is that there are difficulties to maintain the void volumes between the layers that seem to be necessary in order to provide the required flexibility of the material and the required pore volume for absorption.
The term xe2x80x9cmulti-plyxe2x80x9d in this respect means that two or more paper sheets have been combined outside the paper machine in the converting line, such as by embossing and/or gluing, while the term xe2x80x9cmulti-layerxe2x80x9d refers to that two or more sheets of paper have been combined in the paper machine in a wet or moist state, so that papermaking bonds are formed between the layers.
It is further known to impart a three-dimensional pattern in a moist paper web while drying the web. This can be done by so called through-air-drying (TAD), in which hot air is blown through the moist paper web while it is carried by a patterned drying fabric or belt. In connection with the TAD drying the pattern structure of the drying fabric is transferred to the paper web. This structure is essentially maintained also in wet condition of the paper, since it has been imparted to the wet paper web. A description of the TAD technique can be found in e g U.S. Pat. No. 3,812,000.
Through for example WO 99/34055 it is known to impart a three-dimensional a pattern in a moist paper web while drying the web by so called impulse drying and impulse embossing. The wet paper web is passed through a press nip comprising a rotatable roll which is heated and the paper web is given said three-dimensional pattern when passing through the press nip, either by means of a patterned wire and/or by the fact that the heated roll is provided with a pattern intended to be pressed into the paper web against a non-rigid holder-on, such as a felt. The three-dimensional pattern is in this case essentially maintained in wet condition of the paper, since it has been imparted to the wet paper web.
An object of the present invention is to provide a process of joining two or more sheets of paper in wet or moist condition and obtain a final product which possesses properties such as flexibility and absorption capacity which are comparative to those of a multi-ply product. The product could be defined as a single-ply multi-layer product.
This has according to the invention been provided by separately forming and drying at least two paper sheets to a dry content of no more than 80% by weight, imparting a three-dimensional pattern of alternating raised and recessed portions in the paper sheets during drying, combining the at least two paper sheets having a dry content of no more than 80% by weight into a multilayer web, in which void volumes are created between the raised and recessed portions of the combined sheets, drying the multilayer web.
In order to maintain a high bulk it is preferred that drying of the multilayer web is made without any considerable compression of the multilayer web.
The term xe2x80x9cwithout any considerable compressionxe2x80x9d in this respect means that the drying technique used will not cause a compression or bulk reduction of the multilayer web of more than about 25%.
Examples of non-compressible drying techniques are through-air-drying (TAD) and infrared (IR) drying.
According to one embodiment of the invention drying and imparting said three-dimensional pattern in the separately formed paper sheets is made by impulse drying and impulse embossing, wherein the wet paper sheet is passed through a press nip comprising a rotatable roll which is heated so that the paper sheet when passing through the press nip is given said three-dimensional pattern either by means of a patterned wire and/or by the fact that the heated roll is provided with a pattern intended to be pressed into the paper sheet against a holder-on. The holder-on preferably has a non-rigid surface.
In an alternative embodiment drying and imparting said three-dimensional pattern in the separately formed paper sheets is made by through-air-drying (TAD) wherein the wet paper sheet is carried by a patterned wire or belt.
In a further embodiment an additional component, such as an absorbent material and/or spacing means is applied between the paper sheets before combining them.
The invention further refers to a multilayer paper web having a three-dimensional structure, said multilayer paper web comprising at least two paper sheets each of which having a three-dimensional pattern of alternating raised and recessed portions, said paper sheets being joined together point- or spotwise by papermaking bonds forming bonding sites, while leaving void volumes between the sheets and between the bonding sites.